Continuous slurry polymerization volatile removal

ABSTRACT

A process/apparatus is disclosed for continuously separating a liquid medium comprising diluent and unreacted monomers from a polymerization effluent comprising diluent, unreacted monomers and polymer solids, comprising a continuous discharge of the polymerization effluent from a slurry reactor through a discharge valve and transfer conduit into a first intermediate pressure flash tank with a conical bottom defined by substantially straight sides inclined at an angle to that of horizontal equal to or greater than the angle of slide of the slurry/polymer solids and an exit seal chamber of such diameter (d) and length (l) as to maintain a desired volume of concentrated polymer solids/slurry in the exit seal chamber such as to form a pressure seal while continuously discharging a plug flow of concentrated polymer solids/slurry bottom product of the first flash tank from the exit seal chamber through a seal chamber exit reducer with inclined sides defined by substantially straight sides inclined at an angle to that of horizontal equal to or greater than the angle of slide of the polymer solids which remain after removal of about 50 to 100% of the inert diluent therefrom to a second flash tank at a lower pressure.

[0001] This application is a continuation of Ser. No. 09/992,770, filedNov. 6, 2001, which is a continuation-in-part of U.S. application Ser.No. 09/955,729, filed Sep. 19, 2001, which is a divisional applicationof U.S. application Ser. No. 09/679,959, filed Oct. 5, 2000, now U.S.Pat. No. 6,319,997, which is a divisional application of U.S.application Ser. No. 09/313,818, now U.S. Pat. No. 6,204,344, filed May18, 1999, which is a continuation-in-part of U.S. application Ser. Nos.09/080,412 and 09/081,392, now U.S. Pat. No. 6,281,300, both filed May18, 1998, which both claim the benefit of U.S. Provisional ApplicationNo. 60/078,859, filed Mar. 20, 1998. Each application in this chain ofpriority is incorporated by reference herein in its entirety. Each ofthe patent applications listed above through which priority is claimedby this patent application is fully incorporated by reference herein.

FIELD OF INVENTION

[0002] The present invention relates to an apparatus for continuouslyseparating polymer solids from a liquid medium comprising an inertdiluent and unreacted monomers in a slurry polymerization process. Inparticular, the present invention relates to an apparatus forcontinuously separating polymer solids from a liquid medium, drying thepolymer, and recovering the diluent and unreacted monomers with areduction in compression needed for diluent vapor condensation to liquiddiluent for reuse in the polymerization process. In another aspect, theinvention relates to a method for continuously separating polymer solidsfrom a liquid medium. In particular, the invention relates to a methodfor continuously separating polymer solids from a liquid medium, dryingthe polymer, and recovering the inert diluent and unreacted monomers forreuse in the polymerization process.

BACKGROUND OF THE INVENTION

[0003] In many polymerization processes for the production of polymer, apolymerization effluent is formed which is a slurry of particulatepolymer solids suspended in a liquid medium, ordinarily the reactiondiluent and unreacted monomers. A typical example of such processes isdisclosed in Hogan and Bank's U.S. Pat. No. 2,285,721, the disclosure ofwhich is incorporated herein by reference. While the polymerizationprocesses described in the Hogan document employs a catalyst comprisingchromium oxide and a support, the present invention is applicable to anyprocess producing an effluent comprising a slurry of particulate polymersolids suspended in a liquid medium comprising a diluent and unreactedmonomer. Such reaction processes include those which have come to beknown in the art as particle form polymerizations.

[0004] In most commercial scale operations, it is desirable to separatethe polymer and the liquid medium comprising an inert diluent andunreacted monomers in such a manner that the liquid medium is notexposed to contamination so that the liquid medium can be recycled tothe polymerization zone with minimal if any purification. A particularlyfavored technique that has been used heretofore is that disclosed in theScoggin et al, U.S. Pat. No. 3,152,872, more particularly the embodimentillustrated in conjunction with FIG. 2 of that patent. In such processesthe reaction diluent, dissolved monomers, and catalyst are circulated ina loop reactor wherein the pressure of the polymerization reaction isabout 100 to 700 psia. The produced solid polymer is also circulated inthe reactor. A slurry of polymer and the liquid medium is collected inone or more settling legs of the slurry loop reactor from which theslurry is periodically discharged to a flash chamber wherein the mixtureis flashed to a low pressure such as about 20 psia. While the flashingresults in substantially complete removal of the liquid medium from thepolymer, it is necessary to recompress the vaporized polymerizationdiluent (i.e., isobutane) in order to condense the recovered diluent toa liquid form suitable for recycling as liquid diluent to thepolymerization zone. The cost of compression equipment and the utilitiesrequired for its operation often amounts to a significant portion of theexpense involved in producing polymer.

[0005] Some polymerization processes distill the liquefied diluent priorto recycling to the reactor. The purpose of distillation is removal ofmonomers and light-end contaminants. The distilled liquid diluent isthen passed through a treater bed to remove catalyst poisons and then onto the reactor. The equipment and utilities costs for distillation andtreatment can be a significant portion of the cost of producing thepolymer.

[0006] In a commercial scale operation, it is desirable to liquefy thediluent vapors at minimum cost. One such technique used heretofore isdisclosed in Hanson and Sherk's U.S. Pat. No. 4,424,341 in which anintermediate pressure flash step removes a significant portion of thediluent at such a temperature and at such a pressure that this flashedportion of diluent may be liquified by heat exchange instead of by amore costly compression procedure.

BRIEF SUMMARY OF THE INVENTION

[0007] The present invention relates to an apparatus for continuouslyseparating polymer solids from a liquid medium comprising an inertdiluent and unreacted monomers. In another aspect, the invention relatesto an apparatus for continuously separating polymer solids from a liquidmedium, drying the polymer, and recovering the diluent and unreactedmonomers with a reduction in compression needed for diluent vaporcondensation to liquid diluent for reuse in a polymerization process. Inanother aspect, the invention relates to a method for continuouslyseparating polymer solids from a liquid medium. In another aspect, theinvention relates to a method for continuously separating polymer solidsfrom a liquid medium, drying the polymer, and recovering the inertdiluent and unreacted monomers for reuse in a polymerization process.

[0008] In accordance with the present invention, there is provided anapparatus for continuously recovering polymer solids from apolymerization effluent comprising a slurry of the polymer solids in aliquid medium comprising an inert diluent and unreacted monomers. Theapparatus comprises a discharge valve on a slurry reactor, examples ofwhich include slurry loop reactors and stirred tank slurry reactors, forthe continuous discharge of a portion of the slurry reactor contentsinto a first transfer conduit: a first flash tank having a bottomdefined by substantially straight sides inclined at an angle to thehorizontal equal to or greater than the angle of slide of theslurry/polymer solids; wherein the pressure of the first flash tank andtemperature of the polymerization effluent are such that from about 50%to about 100% of the liquid medium will be vaporized and the inertdiluent component of the vapor is condensable, without compression, byheat exchange with a fluid having a temperature in the range of about65° F. to about 135° F.: a first flash tank exit seal chamber,communicating with the first flash tank, of such a length (l) anddiameter (d) as to permit such a level of concentrated polymersolids/slurry to accumulate and form a pressure seal in the first flashtank exit seal chamber: a seal chamber exit reducer providing for acontinuous discharge of a plug flow of concentrated polymersolids/slurry to a second transfer conduit which communicates theconcentrated polymer solids/slurry into a second flash tank wherein thepressure of the second flash tank and temperature of the concentratedpolymer solids/slurry are such that essentially all of any remaininginert diluent and/or unreacted monomer will be vaporized and removedoverhead for condensation by compression and heat exchange and thepolymer solids are discharged from the bottom of the second flash tankfor additional processing or storage.

[0009] The invention provides also a method for the continuous removalof a stream of polymerization effluent from a slurry reactor through adischarge valve; increasing the heat content of the polymerizationeffluent during its transit through the first transfer conduit to atemperature below the fusion point of the polymer while continuouslycommunicating the polymerization effluent to a first flash tank having abottom defined by substantially straight sides inclined at an angle tothe horizontal equal to or greater than the angle of slide of theconcentrated polymer solids/slurry; continuously vaporizing from about50% to about 100% of the liquid medium in the first heated flash tank toyield a concentrated polymer solids/slurry and a vapor stream at such atemperature and pressure that the inert diluent content of the vapor iscondensable, without compression, by heat exchange with a fluid having atemperature in the range from about 65° F. to about 135° F.;continuously discharging the concentrated polymer solids/slurry from thefirst flash tank to a first flash tank exit seal chamber of such alength (l) and diameter (d) that a volume of concentrated polymersolids/slurry is continuously maintained so as to form a pressure sealin the first flash tank exit seal chamber; continuously discharging theconcentrated polymer solids/slurry from the first flash tank sealchamber through a seal chamber exit reducer defined by substantiallystraight sides inclined at an angle to that of horizontal equal to orgreater than the angle of slide of the polymer solids which remain afterremoval of about 50 to 100% of the inert diluent therefrom;communicating a continuous plug flow of concentrated polymersolids/slurry from the first flash tank exit seal chamber through theseal chamber exit reducer to a second transfer conduit whichcommunicates the continuous plug flow of concentrated polymersolids/slurry to a second flash tank; and continuously vaporizingessentially all of any remaining inert diluent and/or unreacted monomerin a second flash tank operated at a lower pressure than the first flashtank; condensing the vaporized inert diluent and/or unreacted monomerfrom the second flash tank by compression and heat exchange; andcontinuously discharging the essentially dried polymer slurry from thesecond flash tank for further processing or storage.

[0010] The present invention also relates to an apparatus for capturinga higher weight percentage of polymer solids from a circulating slurryin a loop reactor than the weight percentage of solids in thecirculating slurry. The apparatus includes a conduit having a first end,wherein the first end extends for a distance into the loop reactor. Theconduit also has portions defining an opening wherein the opening ispositioned relative to the direction of the circulating slurry.Desirably, the opening may be facing the direction of flow of thecirculating slurry. Additionally, a portion of the conduit may extendoutwardly from the loop reactor for discharging, continuously orotherwise the polymer solids from the loop reactor.

[0011] The present invention also provides a process for capturing ahigher weight percentage of polymer solids from a circulating slurry ina loop reactor than the weight percentage of polymer solids in thecirculating slurry. This process includes the step of extending for adistance into a the loop reactor a conduit having portions defining anopening wherein the opening is extends into the circulating slurry.Additionally, this process may include the step of discharging,continuous or otherwise, the polymer solids from the loop reactorthrough a portion of the conduit extending outwardly from the loopreactor.

[0012] The present invention also provides an apparatus for purgingpolymer solids from a conduit connected to a loop reactor and in fluidcommunication with the loop reactor. This apparatus includes a sensor, afirst valve in fluid communication with the conduit, a second valvepositioned between a first inert diluent and the conduit, wherein thefirst inert diluent is in fluid communication with the conduit betweenthe loop reactor and the first valve. In response to a signal producedby the sensor, the first valve is closed and the second valve is openedallowing the first inert diluent to enter the conduit in sufficientquantities and under sufficient pressure to purge polymer solids fromthe conduit. This apparatus may further include a third valve positionedbetween a second inert diluent and the conduit, wherein the second inertdiluent is in fluid communication with the conduit between the loopreactor and the first valve. In this way, when the first valve is openand the second valve is closed the third valve is opened allowing thesecond inert diluent to enter the conduit.

[0013] The present invention also provides a process for purging polymersolids from a conduit connected to a loop reactor and in fluidcommunication with the loop reactor comprising. This process includesthe steps of (i) closing a first valve in response to a first signalfrom a first sensor, wherein the first valve is connected to and influid communication with the conduit, (2) opening a second valve inresponse to a second signal from a second sensor, wherein the secondvalve is fluid communication between a first inert diluent and theconduit, and wherein the first inert diluent is in fluid communicationwith the conduit between the loop reactor and the first valve, and (3)flowing sufficient quantities of the first inert diluent undersufficient pressure into the conduit to purge polymer solids from theconduit. In this process the first and second sensors may be a commonsensor and the first and second signal may be a common signal.

[0014] The present invention also provides an apparatus for returningfines to a polymerization slurry in a loop reactor. The apparatusincludes a discharge valve for discharging a portion of thepolymerization slurry from the loop reactor into a first transferconduit. The first transfer conduit communicates the polymerizationslurry into a first flash tank. The first flash tank converts a portionof the polymerization slurry into a first fluid, such as a vapor. Thefirst fluid includes a portion of the diluent and the fines from thepolymerization slurry. A second transfer conduit communicates the firstfluid to a first cyclone. The first cyclone converts a portion of thefirst fluid into a second fluid, such as a vapor. The second fluidincludes a portion of the diluent and the fines. A third transferconduit communicates the second fluid into a heat exchanger. The heatexchanger converts the second fluid into a liquid comprising the diluentand the fines. A fourth transfer conduit returns the liquid to thepolymerization slurry in the loop reactor. This apparatus may alsoinclude a first transfer conduit heater for heat exchange between thefirst transfer conduit heater and the polymerization slurry.

[0015] The present invention also provides a process for returning finesto a polymerization slurry in a loop reactor. The process includes (i)discharging a portion of the polymerization slurry from the loopreactor, (ii) communicating the discharge polymerization slurry into afirst flash tank, (iii) converting in the flash tank a portion of thepolymerization slurry into a first fluid, the first fluid comprising adiluent and the fines, (iv) communicating the first fluid from the firstflash tank to a first cyclone, (v) converting in the cyclone a portionof the first fluid into a second fluid comprising the diluent and thefines, (vi) communicating the second fluid into a heat exchanger, (vii)converting in the heat exchanger the second fluid into a liquidcomprising the diluent and the fines, and (viii) returning the liquid tothe polymerization slurry in the loop reactor.

[0016] The present invention further provides an apparatus and processfor producing polymer from a polymerization slurry in a loop reactoroperating at a space time yield greater than 2.8 lbs/hr-gal. In thisinstance, the polymer is formed in the polymerization slurry whichincludes a liquid medium and solids. The polymerization slurry isdischarged into a first transfer conduit. The polymerization slurry isreferred to as a polymerization effluent upon leaving the loop reactor.The polymerization effluent is heated in the first transfer conduit to atemperature below the fusion temperature of the polymer solids. Theheated polymerization effluent is communicated through the firsttransfer conduit to a first flash tank. In the first flash tank, fromabout 50% to about 100% of the liquid medium is vaporized. The vapor iscondensed by heat exchange. Polymer solids are discharge from the firstflash tank to a second flash tank through a seal chamber of sufficientdimension such as to maintain a volume of polymer solids in the sealchamber sufficient to maintain a pressure seal. The polymer solids arethen communicated to a second flash tank. In the second flash tank, thepolymer solids are exposed to a pressure reduction from a higherpressure in the first flash tank to a lower pressure in the secondflash. The polymer solids are then discharging from the second flashtank. Additionally, the weight percent of solids in the polymerizationslurry may be greater than 47. The loop reactor may be operated at atotal recirculating pumping head/reactor distance of greater than 0.15ft/ft. The loop reactor may also be operated with a recirculatingpumping head greater than or equal to 200 ft. and have more than eightvertical legs, desirably between 10 and 16 vertical legs, more desirablybetween 10 and 12 vertical legs, most desirably 12 vertical legs. Thevolume of polymerization slurry in the loop reactor may be greater than20,000 gallon.

[0017] The present invention also provides a process for producingpolymer in a continuous slurry loop reactor which comprises: reacting amonomer in a hydrocarbon diluent to form a polymerization slurry ofpolymer solids in a liquid medium; discharging a portion of thepolymerization slurry as effluent which comprises a slurry of dischargedpolymer solids in a discharged liquid medium through a discharge openinginto a first transfer conduit; heating the effluent with a first heater;flashing the effluent in a first flash, wherein at least a portion ofthe discharged liquid medium is vaporized, to form a first flash vaporand a first flash slurry; condensing at least a portion of the firstflash vapor without compression; discharging the first flash slurry fromthe first flash into a second transfer conduit; heating the first flashslurry with a second heater and flashing the first flash slurry in asecond flash. A preferred embodiment further comprises flashing thefirst flash slurry in a second flash to form a second flash vapor andsecond flash polymer solids, wherein at least a portion of the firstflash liquid is vaporized in the second flash. Another embodimentaccording to the present invention comprises condensing at least aportion of the second flash vapor from the second flash. Preferably,about 50 to 100% of the liquid medium of the effluent, more preferablyabout 75 to about 100%, and even more preferably about 95 to about 100%,is vaporized into first flash vapor in the first flash. Also preferably,at least about 50% of the first flash liquid, preferably at least about75%, even more preferably at least about 95%, is vaporized into secondflash vapor in the second flash. In accordance with another embodiment,the loop reactor is operated at about 150-250° F., preferably about175-230° F., more preferably about 200-230° F. In yet anotherembodiment, the loop reactor is also operated at about 400-660 psia,preferably about 500-600 psia, and more preferably about 565 psia.

[0018] In a preferred embodiment in accordance with the presentinvention, the discharging of the effluent into a first transfer conduitis continuous. In another embodiment, the first heater is an in-lineheat exchanger. In another embodiment, the first flash is operated atabout 140 to 315 psia. In yet another embodiment, the second flash isoperated at about 15 to 100 psia.

[0019] Preferably, the first heater heats the effluent to a temperaturebelow the fusion temperature of the polymer solids. Also preferably, theheat input to both in-line heaters is adjusted in such a manner as tosubstantially reduce equipment pluggage, and/or improve the drying ofthe polymer product and/or improve recovery of diluent after dischargeof effluent from the loop reactor.

[0020] Of course, the invention can also include various combinations ofthe embodiments disclosed herein.

[0021] An object of the present invention is to provide both anapparatus and method for the continuous two stage flash drying of thepolymer solids following the continuous removal of the polymerizationeffluent comprising polymer solids and liquid medium comprising inertdiluent and unreacted monomers from a slurry reactor through a pointdischarge valve, a continuous solids level control in the first flashtank exit seal chamber that provides a pressure seal therein whichenables the first flash tank to operate under a substantially greaterpressure than the second flash tank while polymer solids arecontinuously discharged through the seal chamber exit reducer into thesecond transfer conduit and further into the second flash tank whicheliminates plugging in the first flash tank and the continuousliquification of from about 50% to about 100% of the inert diluent vaporby heat exchange rather than compression.

[0022] Another object of the invention is to eliminate the need for asettling leg on the slurry reactor and the intermittent high pressurepulse in the slurry reactor caused by periodic discharging of thecontents of the settling leg. Another object of the present invention isto improve safety by eliminating the possibility of plugging in asettling leg.

[0023] Another object of the invention is to eliminate plugging inequipment downstream from the discharge valve. In a settling leg of apolymerization reactor polymerization continues and the heat of reactionfurther heats the liquid medium and a potential exists for some of thepolymer solids to dissolve or to fuse together. As the contents of thesettling leg exit the discharge valve, the pressure drop causes flashingof some of the liquid medium which results in cooling the remainingliquid medium causing the dissolved polymer to precipitate which tendsto plug downstream equipment. The present invention which eliminates theneed for a settling leg also eliminates this potential for downstreamequipment plugging by avoiding the initial dissolution or fusion of thepolymer solids.

[0024] Another object of the present invention is to increase thereactor through-put by the use of continuous discharge and increasedethylene concentrations in the liquid medium, e.g., greater than orequal to 4 weight percent at reactor outlet, desirably from 4 weightpercent to 8 weight percent, still more desirably from 5 weight percentto 7 weight percent. Settling legs limit ethylene concentrations due toan increased tendency to plug downstream equipment caused by acceleratedreaction within the settling leg. A continuous polymerization effluentslurry flow allows ethylene concentrations to be limited only by theethylene solubility in the liquid diluent in the reactor, therebyincreasing the specific reaction rate for polymerization and increasingreactor throughput.

[0025] Another object of the present invention is to increase the weightpercent (wt %) of polymer solids in the polymerization slurrycirculating in the polymerization zone in the loop reactor. Desirably,the wt % of polymer solids in the polymerization slurry is greater than45, more desirably, from 45 to 65, still more desirably from 50 to 65,and most desirably from 55 to 65.

[0026] Another object of the present invention is to increase the spacetime yield (STY), expressed in terms of pounds per hour-gallon(lbs/hr-gal). Desirably, the STY is greater than 2.6, more desirablyfrom 2.6 to 4.0, and most desirably from 3.3 to 4.0.

[0027] Other aspects, objects, and advantages of the present inventionwill be apparent from the following disclosure and FIGS. 1 and 2.

[0028] The claimed apparatus and process provide several advantages overthe prior art including: (1) allowing for a continuous processing of thecontents of a slurry reactor from the point of discharge of thepolymerization slurry effluent through a discharge valve; a first flashtank; a seal chamber; a seal chamber exit reducer; and therefrom to asecond flash tank, (2) significantly increasing ethylene concentrationin the loop reactor liquid medium thereby increasing reactorthrough-put, (3) significantly increasing the wt % of polymer solids inthe polymerization slurry, (4) significantly increasing reactor spacetime yield and (5) energy consumption is reduced by reducing the need tocompress and/or distill the reactor vapor-liquid effluent. Recyclingcompressors and other downstream equipment can be reduced in size oreliminated.

BRIEF DESCRIPTION OF THE DRAWING

[0029]FIGS. 1 and 2 are a schematic diagram illustrating an apparatusfor continuously separating polymer solids from diluent and unreactedmonomer in accordance with the present invention.

[0030]FIG. 3 is an enlarged, cross sectional view of the dischargeconduit with opening extending a distance into the loop reactor and thecirculating polymerization slurry.

[0031]FIG. 4 is a schematic view of a pressure control system.

[0032]FIG. 5 is a schematic diagram illustrating an embodiment inaccordance with the present invention where two line heaters are usedbefore the flash tanks.

DETAILED DESCRIPTION OF THE INVENTION

[0033] As used herein, the term “polymerization slurry” meanssubstantially a two phase composition including polymer solids andliquid circulating within the loop reactor. The solids include catalystand a polymerized olefin, such as polyethylene. The liquids include aninert diluent, such as isobutane, with dissolved monomer, comonomer,molecular weight control agents, such as hydrogen, antistatic agents,antifouling agents, scavengers, and other process additives.

[0034] As used herein, the term “space time yield” (STY) means theproduction rate of polymer per unit of loop reactor volume orpolymerization slurry volume.

[0035] As used herein, the term “catalyst productivity” means weight ofpolymer produced per weight of catalyst introduced into the loopreactor.

[0036] As used herein, the term “polymer residence time” means theaverage duration that a polymer particle remains within the loopreactor.

[0037] The present invention is applicable to any mixture whichcomprises a slurry of polymer solids and a liquid medium comprising aninert diluent and unreacted polymerizable monomers including slurriesresulting from olefin polymerization. The olefin monomers generallyemployed in such reactions desirably include 1-olefins having from 2 upto 8 carbon atoms per molecule. Typical examples include ethylene,propylene, butene, pentene, hexene and octene. Other examples includevinyl aromatic monomers, like styrene and alkyl-substituted styrene,geminally distributed monomers such as isobutylene and cyclic olefins,such as norbornene and vinyl norbornene. Typical diluents employed insuch olefin polymerizations include saturated aliphatic hydrocarbonshaving 3 to 8, preferably 3 to 4 carbon atoms per molecule, such aspropane, isobutane, propylene, n-butane, n-pentane, isopentane,n-hexane, isooctane, and the like. Of these diluents those of 3 to 4carbon atoms per molecule are preferred, and isobutane is mostpreferred.

[0038] The rate of discharge of the polymerization effluent is such asto allow a continuous process stream from the slurry loop reactor fromthe point of discharge of the liquified polymerization effluent througha single point discharge valve and also through the first flash tank andthe associated vapor recovery and solids recovery systems. The rate ofdischarge of the polymerization effluent is such as to maintain aconstant pressure in the slurry reactor and to eliminate intermittenthigh pressure pulses associated with a discharge of a portion of thereactor contents that occurs with settling legs on slurry reactors.

[0039] The temperature to which the polymerization effluent which isdischarged from the reactor is heated during transit to the first flashtank for vaporization is below the fusion temperature of the polymer.This may be accomplished by appropriate heating of this first transferconduit. The quantity of heat to be supplied to the polymerizationeffluent during its transit through this first conduit to the firstflash tank should preferably be at least equal to that quantity of heatwhich equals the heat of vaporization of that quantity of inert diluentwhich is to be flash vaporized in the first flash tank. This then willprovide for the concentrated polymer solids formed in the first flashtank to be passed to the second flash tank to pass thereto at a highersolids temperature and thus facilitates the removal of residual diluentin the pores of such polymer solids by the operation of the second flashtank. That quantity of heat transferred to the polymerization effluentduring its transit through the first transfer conduit to the first flashtank may even be greater, provided only that the quantity of heat sotransferred will not cause the polymer solids therein to become heatedto such a temperature at which they will tend to fuse or agglomerate onewith another.

[0040] The concentrated polymer solids/slurry are discharged from thefirst flash tank into a first flash tank exit seal chamber of such alength (l) and diameter (d) so as to provide a volume sufficient tomaintain a volume of concentrated polymer solids/slurry sufficient tomaintain a pressure seal in the exit seal chamber. The concentratedpolymer solids/slurry are discharged from the exit seal chamber throughan exit seal chamber reducer to a second transfer conduit whichcommunicates the concentrated polymer solids/slurry as a plug flow to asecond flash tank. The exit seal chamber reducer is defined bysubstantially straight sides inclined at an angle to that of horizontalequal to or greater than the angle of slide of the concentrated polymersolids/slurry.

[0041] The pressure for the first flash step will vary depending on thenature of the diluent and unreacted monomers and the temperature of thepolymerization effluent. Typically, pressures in the range of from about140 psia to about 315 psia can be employed; more preferably from about200 psia to about 270 psia; and most preferably from about 225 psia toabout 250 psia.

[0042] The heat exchanging fluid used to condense the vapor from thefirst flash step is at a temperature in the range of from about 65° F.to about 150° F. A preferred embodiment uses a heat exchange fluid at atemperature of from about 75° F. to about 140° F. A most preferredembodiment uses a heat exchange fluid at a temperature of from about 85°F. to about 130° F.

[0043] A further understanding of the present invention will be providedby referring to FIG. 1 which illustrates a system comprising anembodiment of the invention.

[0044] In the embodiment illustrated in FIG. 1, the polymerization iscarried out in a loop reactor 1. It will be understood that while theloop reactor 1 is illustrated with four vertical legs, the loop reactor1 may be equipped with more legs, desirably eight or more legs,desirable between 8 and 20, more desirable between 8 and 16, mostdesirable with 12 legs. The polymerization slurry is directionallycirculated throughout the loop reactor 1 as illustrated by arrows A-D byone or more pumps, such as axial flow pumps, 2A and 2B. Desirably, theloop reactor 1 is equipped with multiple pumps wherein each pump isdedicated to an even number of legs, such as for example, four legs, sixlegs, eight legs, etc. Diluent comonomer and monomer are introduced intothe loop reactor 1 from the diluent storage vessel 40, the comonomerstorage vessel 41, and the monomer source 42 through their respectivetreater beds 37, 38, and 39 through conduits 5, 4 and 3, respectively,connected to conduit 6. Catalyst is added to the loop reactor 1 throughone or more catalyst feed systems 7A and 7B. Normally, catalyst isintroduced in a hydrocarbon diluent.

[0045] Polymerization slurry may be removed from the loop reactor bycontinuous discharge through a discharge conduit 8A. It will beunderstood that the loop reactor 1 may be equipped with one or moredischarge conduits 8A. It will be also understood that the dischargeconduit(s) 8A may be operated in a continuous or discontinuous mode, butdesirably a continuous mode. The discharge conduit 8A extends for adistance through a portion of the wall of the loop reactor 1 and intothe circulating polymerization slurry. By extending for a distance intothe polymerization slurry, the discharge conduit 8A may removepolymerization effluent from the circulating polymerization slurry overan area defined from near or adjacent the inside wall of the loopreactor 1 to a distance extending into the circulating polymerizationslurry. In this way, a higher weight percentage of polymer solids may beformed within the conduit 8A and ultimately removed from the loopreactor 1 than the weight percentage of polymer solids within theotherwise circulating polymerization slurry. A pressure control system(not shown in FIG. 1) operates in concert with the discharge conduit 8A.The discharge conduit 8A and the pressure control system 410 are moreclearly illustrated in FIGS. 3 and 4 and will be discussed in greaterdetail below.

[0046] The polymerization effluent passes from the discharge conduit 8Ato the discharge valve 8B to a conduit 9 which is provided with a lineheater 10 and into the first flash tank 11 which separates vaporizedliquid medium from polymer slurry/solids. Conduit 9 has an indirect heatexchange means such as a flash line heater 10.

[0047] Vaporized liquid medium comprising diluent and unreacted monomersexit the first flash tank 11 via transfer conduit 12 through which it ispassed into a separator, such as a cyclone, illustrated by referencenumber 13 which separates entrained polymer solids from the vapor.Polymer solids separated by the cyclone 13 are passed via conduit 14through a dual valving assembly 14A designed to maintain a pressure sealbelow cyclone 13 to a lower pressure second flash tank 15.

[0048] The dual valving assemble 14A includes valves 14B and 14C. Thevalving assemble 14A in conjunction with conduit 14 operate toperiodically discharge polymer solids which have collected in theconduit 14 from the cyclone 13. The valving assembly 14A also maintainsthe pressure differential between the higher pressure environment in thecyclone 13 and the lower pressure environment in the second flash tank15. In the operation of the valving assembly 14A, valves 14B and 14C aresequentially opened and closed. At the beginning of this sequence, thevalve 14B is open and the valve 14C is closed allowing the polymersolids from the cyclone 13 to collect in the conduit 14. Upon thepassage of time and/or the collection of sufficient polymer solids inthe conduit 14, the valve 14B closes capturing a portion of the highpressure environment from the cyclone 13 in the conduit 14. After thevalve 14B closes, the valve 14C opens and the polymer solids collectedin the conduit 14 are forcibly discharged into the flash tank 15 by thedifferential pressure between the higher pressure environment in conduit14 and the lower pressure environment in the flash tank 15. Afterdischarging the polymer solids from conduit 14 into the flash tank 15,the valve 14C closes. Once the valve 14C closes, the valve 14B is openedat which time polymer solids will again collect in conduit 14 from thecyclone 13. The above sequence is then repeated.

[0049] Referring back to the first flash tank 11, the concentratedpolymer solids/slurry in the bottom of the first flash tank 11continuously settles by sliding along the straight line bottom surface16 thereof into the seal chamber 17 which is illustrated in enlargementFIG. 2. A polymer solids/slurry level 43 is maintained in the sealchamber 17 to eliminate plugging tendencies in first flash tank 11 andto form a pressure seal so that the first flash tank 11 can operate at asubstantially higher pressure than the second flash tank 15. Polymerslurry/solids are continuously discharged from the seal chamber 17 intothe lower pressure second flash tank 15. The length (l), diameter (d),and volume of the seal chamber 17 and the geometry of the seal chamberexit reducer 18 are chosen so as to provide a variable residence timeand provide a continuous plug flow of concentrated polymer solids/slurryto minimize “dead” space and reduce plugging tendencies. The sealchamber 17 length must be sufficient to allow particle (polymer solids)level measurement and control.

[0050] Particle level measurement and control may be accomplished by anuclear level indicating system 18D. The nuclear level indicating system18D includes a nuclear radiating source (not shown) and receiver orlevel element 18A in signal communication with a level indicatingcontroller 18B. In operation, the level element 18A generates a signalproportional to the particulate level in the seal chamber 17. Thissignal is conveyed to the level indicating controller 18B. In responseto this signal and a preset value, the level indicating controller 18Bsends a signal through a conduit (illustrated by broken line 18C) to acontrol valve 18E which selectively controls the discharge of polymersolids into a conduit 19.

[0051] Typical residence times of the concentrated polymer solid/slurryin the seal chamber 17 are from 5 seconds to 10 minutes, preferableresidence times are from 10 seconds to 2 minutes and most preferableresidence times from 15-45 seconds. The continuous plug flow ofconcentrated polymer solids/slurry forms a pressure seal wherein theconcentrated polymer solids/slurry have an l/d ratio inside the sealchamber 17 which is typically 1.5 to 8, preferable l/d is 2 to 6 andmost preferable is 2.2 to 3. Typically the seal chamber exit reducer 18sides are inclined, relative to the horizontal, 60-85 degrees,preferable 65-80 degrees and most preferable 68-75 degrees. The sealchamber exit reducer 18 geometry is defined by substantially straightsides inclined at an angle to that of horizontal equal to or greaterthan the angle of slide of the concentrated polymer slurry/solids andcommunicates the concentrated polymer solid/slurry to a second transferconduit 19 which communicates with a feed inlet of flash tank 15. Inflash tank 15 substantially all of any remaining inert diluent andunreacted monomer in the concentrated polymerization effluent isvaporized and taken overhead via conduit 20 to a second cyclone 21.

[0052] One embodiment in accordance with the present invention utilizesmultiple heaters: at least one heater before each flash. In a preferredembodiment as schematically illustrated in FIG. 5, two flash tanks 11and 15 are utilized, each having an in-line heater associated therewith.According to a preferred embodiment, the polymerization effluent fromthe loop reactor, which comprises polymer solids and liquid medium, isheated, preferably, in first transfer conduit 9, with a first in-lineheater 10 before first flash, illustrated in FIG. 5 as flash tank 11. Afirst flash vapor and a first flash slurry are formed in the first flashand at least a portion of the first flash vapor from the first flash isthen condensed without compression. As used herein, “flash slurry” meanspolymer solids containing entrained (absorbed) liquid medium andentrained flash vapor (if any) and/or such polymer solids slurried in“free-flowing” liquid medium. The first flash slurry from the firstflash 11 is discharged into a second transfer conduit 19 and is heated,preferably in the second transfer conduit, with a second in-line heater10A before the second flash 15. A second flash vapor and second flashpolymer solids are formed in the second flash 15 and, preferably, atleast a portion of the second flash vapor from the second flash tank iscondensed.

[0053] In a preferred embodiment in accordance with the presentinvention, about 50 to about 100% of the liquid medium of the effluentfrom the loop reactor is vaporized into first flash vapor in the firstflash, preferably about 75 to about 100%, and more preferably about 95to about 100%. At least about 50% of first flash liquid is vaporizedinto second flash vapor in the second flash, preferably at least about75%, and more preferably at least about 95%.

[0054] According to another embodiment in accordance with the presentinvention, the loop reactor is operated at about 150-250° F., preferablyabout 175-230° F., and more preferably about 200-230° F. The reactorpressure is preferably at about 400-660 psia, preferably about 500-600psia, and more preferably about 565 psia. Although these ranges aregiven as a list of upper preferable values and lower preferable values,this is to be understood as specifically disclosing all ranges formedfrom any pair of an upper preferred value and a lower preferred value,regardless whether ranges are separately disclosed. For instance, theloop reactor can be operated at about 150-230° F.

[0055] The discharging from the loop reactor into the first transferconduit 9 is preferably continuous. Although any heaters can be used, itis preferred that the heaters are in-line heat exchangers. The firstheater preferably heats the effluent to a temperature below the fusiontemperature of the polymer solids and preferably at least a portion ofthe liquid medium of the effluent is vaporized in the first flash. Afterthe desired amount of the liquid medium is vaporized in the first flash,some amount of liquid medium and some amount of vapor will remainentrained in the polymer solids. For example, it is estimated that about2-4% of the liquid medium is entrained in the polymer solids.

[0056] Using the two in-line heaters, one before each of the flashtanks, has additional operational benefits, as will readily occur to oneof ordinary skill in the art having the benefit of the presentdisclosure. These benefits include, but are not limited to 1)substantial reduction of equipment pluggage, 2) improvement in dryingthe polymer product, and 3) improvement in recovering diluent.

[0057] In the preferred two-heater system, the pressure for the firstflash varies depending on the nature of the liquid medium and un-reactedmonomers and the temperature of the polymerization effluent. Pressuresin the range of about 140 to 315 psia can used, preferably about 160 to270 psia, more preferably about 170 to 200 psia. Due to the use of asecond heater before the second flash 15 in the preferred embodiment tovolatilize the entrained liquid medium from the polymer solids, thesecond flash can be operated at a higher pressure than the operatingpressure of the second flash in a one-heater system. For instance, thesecond flash can be operated at about 15 to 100 psia.

[0058] The heat input into the first in-line heater 10 is adjustedaccording to at least one process parameter, preferably flash tankpressure. This one process parameter can be selected to achieve any orall of the additional benefits this embodiment brings but preferably tosubstantially reduce equipment pluggage. The heat input into the firstin-line heater 10 is an amount sufficient to vaporize the desired amountof the liquid medium. Preferably, the heat input into the first in-lineheater is an amount sufficient to vaporize substantially all“free-flowing” liquid thus leaving only entrained liquid and vapor, ifany, in the polymer solids. If the effluent is overheated at this stage,equipment pluggage will occur at the first flash.

[0059] Referring now to the cyclone 13, the major portion of the liquidmedium in the polymerization effluent may be been taken to cyclone 13 asvapor. The vapor after having a portion of the entrained catalyst andpolymer solids removed is passed via conduit 22 through a heat exchangersystem 23A wherein the vapor at a pressure from about 140 psia to about315 psia is condensed by indirect heat exchange with a heat exchangefluid such as to eliminate the need for compression. The portion of theentrained catalyst and polymer solids not removed by the cyclone 13 aregenerally smaller in size and may be referred to as “fines”, “polymerfines” and/or “catalyst fines”. These fines generally include unreactedand/or under-reacted catalyst.

[0060] The heat exchanger system 23A includes a heat exchanger 23E and atempered water circulating pump 23B connected to the heat exchanger 23Eby conduit 23C. A tempered water temperature control valve 23D isconnected to the heat exchanger 23E and water circulating pump 23B byconduits 23F and 23G, respectively. Cooling water from a cooling watersource (not shown) is conveyed via a cooling water conduit 23H into theconduit 23G between the control valve 23D and the circulating pump 23B.A temperature indicating controller (TIC) 23J is connected between thecontrol valve 23D and the conduit 23C. Between the controller 23J andthe conduit 23C resides a temperature element 23K.

[0061] The heat exchanger system 23A operates to control the amount ofvapor condensed in the heat exchanger 23E. This is accomplished bycontrolling the flow of cooling water introduced into the conduit 23Gfrom the conduit 23H by exhausting heated water formed in the heatexchanger 23E. The heated water from the heat exchanger 23E is conveyedto the control valve 23D via the conduit 23F. The heated water exits thecontrol valve 23D via the conduit 231.

[0062] More specifically, cooling water from the conduit 23H enteringthe conduit 23G mixes with circulating tempered water in the conduit23G, the mixture thereof enters the pump 23B. The water exiting the pump23B enters the conduit 23C, a portion of which contacts the temperatureelement 23K, in route to the heat exchanger 23E. The temperature element23K generates an signal proportional to the temperature in conduit 23C.The signal is conveyed to the temperature indicating controller 23J. Inresponse to this signal and a preset temperature value, the temperatureindicating controller 23J sends a signal through a signal conduit(illustrated by the broken line 23L) to the control valve 23D whichselectively controls the volume of heated water exiting the heatexchanger system 24A through the conduit 23I.

[0063] The condensed liquid medium formed at the heat exchanger 23Eincludes diluent, unreacted/under-reacted catalyst, polymer solids andunreacted monomers. This condensed liquid medium is then passed to anaccumulator 24B via a conduit 22A.

[0064] It is desirable to control the amount of vapor condensed in theheat exchanger 23E and to maintain sufficient vapor pressure in theaccumulator 24B. In this way, a pressure control valve 24A can maintainsufficient back pressure on the accumulator 24B. By maintaining asufficient back pressure on the accumulator 24B, a proper operatingpressure is maintained in the first flash tank 11. The pressure controlvalve 24A is actuated by a pressure indicating controller 24C in concertwith a pressure element 24D. The pressure element 24D is in sensingcommunication with the accumulator 24B. The pressure element 24Dgenerates an signal proportional to the pressure in the accumulator 24B.In response to this signal and a preset pressure value, the pressureindicating controller 24C sends a signal through a signal conduit(illustrated by the broken line 24E) to the control valve 24A whichselectively controls the back pressure on the accumulator 24B.

[0065] A pump 25 is provided for conveying the condensed liquid mediumfrom the accumulator 24B back to the polymerization zone by a conduit26. In this way, the unreacted/under-reacted catalyst and polymer solidsnot removed by the cyclone 13 are returned for further polymerization tothe loop reactor 1.

[0066] The polymer solids in the lower pressure second flash tank 15 arepassed via a conduit 27 to a conventional dryer 28. The vapor exitingthe secondary cyclone 21, after filtration in a filter unit 29, ispassed by a conduit 30 to a compressor 31 and the compressed vapors arepassed through a conduit 32 to a condenser 33 where vapor is condensedand the condensate is passed through conduit 34 to storage vessel 35.The condensed liquid medium in the storage vessel 35 is typically ventedoverhead for removal of light-end contaminants. The inert diluent can bereturned to the process through a treater bed 37 to remove catalystpoisons or distilled in unit 36 for more complete removal of light-endsand then returned to the process through a treater bed.

[0067] Turning now to FIG. 3, a portion of a wall 310 of the loopreactor 1 through which the discharge conduit 8A extends is illustrated.The discharge conduit 8A may extend into the reactor at various angles.Desirably, the discharge conduit 8A extends into the loop reactor atsubstantially a right angle relative to the wall 310.

[0068] The wall 310 includes an inside surface 312 and an outsidesurface 314. The inside surface 312 supports the circulatingpolymerization slurry illustrated by directional arrows 318. Thedischarge conduit 8A has a top 316A, and a continuous side 316B.Portions of the side 316B define an opening 320. The opening 320 has avertical opening dimensions v1 and v2 defined by walls 320A and 320B ofthe side 316B. Desirably, the v1 dimension is greater than the v2dimension. The opening 320 has horizontal opening dimensions h1 and h2(not shown). The opening 320 may be formed in any suitable shape, suchas rectangular, oval, or a combination thereof. In one embodiment, theopening 320 may be conical-shaped or scooped shaped.

[0069] The opening 320 communicates with a channel 322 defined by theinside surfaces of the top 316A and the side 316B. The channel 322conveys captured polymerization slurry, illustrated by directional arrow324 to the discharge valve 8B (not shown).

[0070] The opening 320 is sized and positioned relative to the directionof movement of the circulating polymerization slurry 318. Desirably, theopening 320 is in a substantially facing position to the direction ofthe circulating polymerization slurry 318. More desirably, the opening320 faces the direction of the circulating slurry 318. In this way, aportion of the polymerization slurry 324 containing polymer solids isremoved from the circulating polymerization slurry 318 over an area fromnear or adjacent the inside wall 312 of the loop reactor 1 to a distanceextending into the circulating polymerization slurry 318. In this way, ahigher weight percentage of polymer solids may be formed within theconduit 8A than the weight percentage of polymer solids within theotherwise circulating polymerization slurry.

[0071] This weight percentage increase of polymer solids may depend uponthe location of the discharge conduit 8A along the loop reactor 1, theinsertion depth of the discharge conduit 8A within the loop reactor, thesize and configuration of the opening 320, the orientation of theopening 320 relative to the direction of the circulating polymerizationslurry, and the weight percentage of polymer solids in the circulatingpolymerization slurry 318. For example, between 1 to 5 weight percentagecalculated increase is observed with a discharge conduit 8A having an v1dimension of approximately 5 inches and a h1 dimension of approximately1 inch. The discharge conduit 8A was positioned 10 ft downstream of a 90degree bend in the loop reactor 1 in a portion of the loop reactor wall314 adjacent the ground. The discharge conduit 8A extended approximately5.5 inches into the circulating polymerization slurry stream. Thevelocity of the circulating polymerization slurry was in the range of 28to 34 ft/sec with weight percent of polymer solids in the range of 48 to53.

[0072] Turning now to FIG. 4, the pressure control system 410 isillustrated. The pressure control system 410 operates to maintainsubstantially uniform pressure within the loop reactor 1 by controllingthe discharge of polymerization effluent from the loop reactor 1 via thedischarge conduit 8A. The control system 410 also operates to preventplugging of the discharge conduit 8A by polymer solids during pressurefluctuations within the loop reactor 1 and/or when the flow ofpolymerization effluent from the discharge conduit 8A to conduit 9 isinterrupted and/or stopped.

[0073] The pressure control system 410 includes a first inert diluentsource 412, such as isobutane, and an inert diluent conduit 414 incommunication with a loop reactor conduit 416. The flow of inert diluentthrough the inert diluent conduit 414 to the loop reactor conduit 416 iscontrolled by the control valve 418 in concert with a flow element 420and a flow indicator controller 422. The purpose of metering the flow ofinert diluent from the first inert diluent source 412 to the loopreactor 1 is to prevent plugging of the conduit 416 by polymer solids.In this way, a loop reactor pressure element 441 (discussed below), incommunication with the loop reactor conduit 416, may more accuratelymonitor the pressure in the loop reactor 1.

[0074] The pressure control system 410 further includes as second inertdiluent source 424 and a third inert diluent source 426. Inert diluent,such as isobutane, from the second inert diluent source 424 flows into aconduit 428 towards a control valve 430 which is in fluid communicationwith a conduit 432. The control valve 430, in concert with a flowelement 431 and a flow indicator controller 433, meters the flow ofinert diluent from the second inert diluent source 424 into conduit 432.The conduit 432 is in fluid communication with a conduit 434 and thedischarge conduit 8A, terminating in the discharge conduit 8A at a pointbetween the loop reactor 1 and the discharge valve 8B. The purpose ofmetering the flow of inert diluent from the second inert diluent source422 into the conduit 432 is to prevent plugging of the conduit 432 bypolymer solids which might otherwise back flow into the conduit 432 fromthe discharge conduit 8A. Additionally, the flow of inert diluent fromthe second inert diluent source 422 also prevents plugging of theconduit 434 and the control valve 440 by polymer solids which might backflow into conduit 432 from the discharge conduit 8A.

[0075] Inert diluent from the third inert diluent source 426 flows intoa conduit 438 towards a control valve 440 which is in fluidcommunication with conduit 434. As will be explained in greater detailbelow, in the event of a sufficient pressure fluctuation within the loopreactor 1, the control valve 440 operates to initiate a sufficient flowunder sufficient pressure of inert diluent from the third inert diluentsource 426 to purge and/or discharge polymer solids from the dischargeconduit 8A into the loop reactor 1. In this instance, generally the flowof inert diluent from the third inert diluent source 426 into theconduit 432 will be greater than the flow of inert diluent from thesecond inert diluent source 424 into the conduit 432. For example, theflow of inert diluent from the second inert diluent source 424 to thedischarge conduit 8A may be in a range of 0.5 to less than 2.0gallons/min. The flow of inert diluent from the third inert diluentsource 426 to the discharge conduit 8A may be in a range of 2.0 to 20gallons/min.

[0076] The loop reactor pressure element 441 and a pressure indicatingcontroller 442 perform several functions. As previously mentioned, thepressure element 441 monitors the loop reactor 1 pressure via theconduit 416. In response to this pressure, the loop reactor pressureelement 441 generates an signal proportional to the pressure in conduit416. This signal is conveyed to the pressure indicating controller 442.In response to this signal and a preset pressure value, the pressureindicating controller 442 sends a signal through a signal conduit(illustrated by the broken line 444) to the discharge valve 8B and thecontrol valve 440.

[0077] During normal loop reactor operations, the discharge valve 8B ispositioned to permit the flow of polymerization effluent from thedischarge conduit 8A to conduit 9. At the same time, the control valve440 is closed preventing the flow of inert diluent from the third inertdiluent source 426 to the discharge conduit. When sufficient pressurefluctuations occur and/or when partial depressurization in the loopreactor 1 are detected by the loop reactor pressure element 441, thesignal generated by the pressure indicating controller 442 causes thedischarge valve 8B to close and the control valve 440 to open. Byclosing discharge valve 8B, thus interrupting the discharge from theloop reactor 1, pressure within the loop reactor 1 may be restored. Byopening the control valve 440 and flowing sufficient volumes of inertdiluent from the third inert diluent source 426 into the dischargeconduit 8A under sufficient pressure, polymer solids remaining in thedischarge conduit 8A between the discharge valve 8B and the loop reactor1 may be flushed out of and/or purged from the discharge conduit 8A andinto the loop reactor 1. Additionally, by maintaining a sufficient flowof inert diluent, continuous or otherwise, into and/or through thedischarge conduit 8A while the discharge valve 8B is closed, the polymersolids within the loop reactor 1 are prevented from entering and/orsubstantially collecting in the discharge conduit 8A and/or plugging thedischarge conduit 8A. Upon return of normal operations, the controlvalve 440 closes terminating the flow of inert diluent from the thirdinert diluent source 426 and the discharge valve 8B opens to resume theflow of polymerization effluent through the discharge conduit 8A intothe conduit 9.

[0078] Having broadly described the present invention it is believedthat the same will become even more apparent by reference to thefollowing examples. It will be appreciated that the examples arepresented solely for the purpose of illustration and should not beconstrued as limiting the invention.

EXAMPLES Example 1

[0079] A typical ethylene polymerization process can be conducted at atemperature of about 215° F. and a pressure of 565 psia. An example ofsuch a process would result in a polymerization effluent of about 83,000pounds per hour comprising about 45,000 pounds per hour of polyethylenepolymer solids and about 38,000 pounds per hour of isobutane andunreacted monomers. The continuously discharged polymerization effluentis flashed in the first flash tank at a pressure of about 240 psia and atemperature of about 180° F. to remove overhead about 35,000 pounds perhour of diluent and unreacted monomer vapors and entrained particulates.Auxiliary heat to impart an additional quantity of heat to thepolymerization effluent is supplied by appropriate heating means duringthe transit between the discharge valve and the first flash tank. Afterremoval of the fines, the isobutane vapor is condensed, withoutcompression, by heat exchange at a pressure of about 240 psia and atemperature of about 135° F. The polymer slurry/solids discharging fromthe bottom of the first flash tank into the seal chamber form acontinuous plug flow of concentrated polymer slurry/solids, whichprovides a pressure seal, with an l/d ratio of the plug of polymerslurry/solids of 2.5 in an 8′4″ long seal chamber having an l/d ratio of5.5 and with a cone angle of about 68° on the seal chamber exit reducer.The residence time of the continuous plug flow of concentrated polymerslurry/solids is about 16 seconds. The concentrated polymerslurry/solids are continuously discharged from the bottom of the firstflash tank at a temperature of about 180° F. and a pressure of about 240psia through a seal chamber, seal chamber exit reducer, and a secondtransfer conduit into a feed inlet on a second flash tank. The remainingliquid medium in the concentrated polymer slurry/solids communicated tothe second flash tank is flashed at a temperature of about 175° F. andat a pressure of about 25 psia to remove about 4,300 pounds per hour ofisobutane and unreacted monomers which are condensed by compression andheat exchange.

Example 2

[0080] A typical ethylene polymerization process can additionally beconducted at a temperature of about 215° F. and a pressure of 565 psia.An example of such a process would result in a polymerization effluentof about 83,000 pounds per hour comprising about 45,000 pounds per hourof polyethylene polymer solids and about 38,000 pounds per hour ofisobutane and unreacted monomers. The continuously dischargedpolymerization effluent is flashed in the first flash tank at a pressureof about 240 psia and a temperature of about 175° F. to remove overheadabout 23,000 pounds per hour of diluent and unreacted monomer vapors andentrained particulates. After removal of the fines, the isobutane vaporis condensed, without compression, by heat exchange at a pressure ofabout 240 psia and a temperature of about 112° F. The polymerslurry/solids discharging from the bottom of the first flash tank intothe seal chamber form a continuous plug flow of concentrated polymerslurry/solids, which provides a pressure seal, with an l/d ratio of theplug of polymer slurry/solids of 2.5 in an 8′4″ long seal chamber withan l/d ratio of 5.5 and with a cone angle of about 68° on the sealchamber exit reducer. The residence time of the continuous plug flow ofconcentrated polymer slurry/solids in the seal chamber is about 16seconds. About 60,000 pounds per hour of concentrated polymerslurry/solids are continuously discharged from the bottom of the firstflash tank at a temperature of about 175° F. and a pressure of about 240psia through a seal chamber, seal chamber exit reducer and a secondtransfer conduit into a feed inlet on a second flash tank. The remainingliquid medium in the concentrated polymer slurry/solids communicated tothe second flash tank is flashed at a temperature of about 125° F. andat a pressure of about 25 psia to remove about 16,000 pounds per hour ofisobutane and unreacted monomer which are condensed by compression andheat exchange.

Example 3

[0081] An example of a typical ethylene polymerization process wascarried out in an eight leg, 20 inch reactor with settling legs havingan overall length of 833 ft and a volume of 11,500 gallons. The reactorwas equipped with a single flash tank (requiring 100% compression of alldiluent discharged from the reactor), a single 460-480 kilowattcirculating pump having a pump head in the range from 85 ft to 110 ft,producing a circulation rate in the range from 21,000 to 28,000 gallonsper minute (gpm) and operated in a discontinuous discharge mode. Thepolymerization temperature and pressure in the reactor would be betweenabout 215° F. to 218° F. and a pressure of 565 psia.

[0082] In the process of example 3, the reactor slurry density is in therange from 0.555 gm/cc to 0.565 gm/cc, a polymer production rate rangefrom 28,000 pounds to 31,000 pounds per hour while maintaining a reactorsolids concentration weight percentage in the range from 46 to 48 with apolymer residence time in the range from 0.83 to 0.92 hours. Space timeyield (STY) was in the range from 2.4 to 2.7. Example 3 data and resultsare further illustrated in Table 1.

Example 4

[0083] Another example of a typical ethylene polymerization processillustrating high polymer solids loading was carried out in an eightleg, 20 inch reactor having an overall length of 833 ft and a volume of11,500 gallons. The reactor in example 4 was equipped dual flash tanks,single discharge conduit, two circulating pumps in series consuming atotal of between 890 and 920 kilowatts producing a total pumping head inthe range from 190 ft to 240 ft, producing a circulation rate in therange from 23,000 to 30,000 gpm and operated in a continuous dischargemode. The polymerization temperature and pressure in the reactor wouldbe between about 217° F. to 218° F. and a pressure of 565 psia.

[0084] In the process of example 4 a polymerization effluent wasproduced having a reactor slurry density in the range from 0.588 to0.592 gm/cc, a polymer production rate in the range from 38,000 to42,000 pounds per hour while maintaining a reactor solids concentrationweight percentage in the range of 54 to 57 with a polymer residence timein the range of 0.68 to 0.79 hours. Space time yield (STY) was in therange of 3.3 to 3.7. Example 4 data and results are further illustratedin Table 1.

[0085] The continuously discharged polymerization effluent is flashed inthe first flash tank at a pressure of about 240 psia and a temperatureof about 175° F. to remove overhead about 16,000 pounds per hour ofdiluent and unreacted monomer vapors and entrained particulates. Afterremoval of the fines, the isobutane vapor is condensed, withoutcompression, by heat exchange at a pressure of about 240 psia and atemperature of about 112° F. The polymer slurry/solids discharging fromthe bottom of the first flash tank into the seal chamber form acontinuous plug flow of concentrated polymer slurry/solids, whichprovides a pressure seal, with an l/d ratio of the plug of polymerslurry/solids of 2.5 in an 8′4″ long seal chamber with an l/d ratio of5.5 and with a cone angle of about 68° on the seal chamber exit reducer.The residence time of the continuous plug flow of concentrated polymerslurry/solids in the seal chamber is about 16 seconds. Concentratedpolymer slurry/solids are continuously discharged from the bottom of thefirst flash tank at a temperature of about 175° F. and a pressure ofabout 240 psia through a seal chamber, seal chamber exit reducer and asecond transfer conduit into a feed inlet on a second flash tank. Theremaining liquid medium in the concentrated polymer slurry/solidscommunicated to the second flash tank is flashed at a temperature ofabout 125° F. and at a pressure of about 25 psia to remove about 16,000pounds per hour of isobutane and unreacted monomer which are condensedby compression and heat exchange. TABLE 1 ETHYLENE POLYMERIZATION DATAEXAMPLE 3 EXAMPLE 4 Nominal pump(s) size, inches 20 20 Reactor solidsconcentration, wt. % 46-48 54-57 Polymer production rate, K lbs./hr.28-31 38-42 Reactor circulation pump power, KW 460-480 890-920Circulation pump head, ft.  85-110 190-240 Circulation rate, GPM21,000-28,000 23,000-30,000 Reactor slurry density, gm/cc 0.555-0.5650.588-0.592 Reactor temperature, degrees F 215-218 217-218 Ethyleneconcentration, wt. % 4.0-4.4 5.0-6.0 Hexene concentration, wt. %0.13-0.19 0.13-0.19 Heat transfer coefficient, btu/hr-f-ft 215-225230-245 Reactor volume, gallons 11,500 11,500 Reactor length, ft.   833  833 Circulating pump head 0.100-0.132 0.228-0.288 per reactor length,ft/ft Catalyst productivity, lb/lb 2,700-3,000 2,700-3,000 Polymerresidence time, hrs. 0.83-0.92 0.68-0.79 Space time yield, lbs/hr - gal2.4-2.7 3.3-3.7 Isobutane compressed and recycled, %   100 45-60

[0086] Discussion

[0087] In view of the above description and examples, severalobservations relative to the apparatus and process can be made.

[0088] It has been observed that by increasing the head and flowcapability of the loop reactor circulating pump(s), higher weightpercent solids can be circulated in the reactor. It has also beenobserved that attaining the necessary head and flow from one pump isincreasingly difficult as percent solids are increased above 45 weightpercent and/or reactor length is increased. Therefore, the use of twopumps in series allows a doubling of pumping head capability and aresulting percent solids increase. Increased weight percent solids inthe loop reactor increases catalyst residence time, which for chromeoxide and Ziiegler-Natta catalysts, increases catalyst productivity. Onecan choose to take advantage of higher percent solids and longerresidence time by keeping production rate constant at reduced catalystfeed rate and improve the catalyst yield. Another alternative is tomaintain catalyst feed rate constant and increase the reactor throughputand therefor increase STY at nearly constant catalyst productivity.Higher solids also increases the weight percent solids removed from thereactor which reduces isobutane processing cost in recycle equipment.Desirably, the higher solids are removed continuously. Continuousdischarge may occur through a single point discharge line.

[0089] In a loop reactor, it is not always possible to locate thecontinuous discharge line in an optimal location to take advantage ofcentrifugal force to increase the weight percent solids and thereforereduce the amount of isobutane entrained with the polymer solids. It hasbeen observed that a specifically designed pipe as illustrated in FIG. 3inserted into the loop reactor can increase weight percent solidsremoved from the reactor. This pipe insert will function in any sectionof the loop reactor and in a straight section will increase the weightpercent solids to that equal to that in a location which takes advantageof centrifugal force to concentrate solids.

[0090] With the development of high weight percent solids circulationcapability in the loop reactor and two-stage flash, the need toconcentrate solids in the reactor discharge is reduced compared to theconventional loop reactor operations having low solids circulation,single-stage flash, continuous discharge line, and continuous dischargeor otherwise. Therefore, the conventional loop reactor settling legs,which are designed to maximize polymer solids concentration prior todischarge, can be replaced with a continuous discharge line, whichsimplifies the system mechanically, reduces capital cost, improvessafety, reduces maintenance and improves reactor control. Settling legsrequire routine maintenance due to their plugging tendency and can formmaterial which plugs downstream polymer handling equipment. Maximum loopreactor ethylene concentration is limited by settling legs due to thetendency for polymer to grow in the legs at elevated ethyleneconcentrations between discharges and therefore plug the leg. Continuousdischarge eliminates this tendency. Another advantage of continuousdischarge is better response to a sudden drop in reactor pressure, whichcan happen if ethylene flow is quickly reduced. Under this condition,settling legs will stop discharging and can plug with polymer withinminutes

[0091] A development which would increases efficiency of the two-stageflash system is the continuous flash line heater. The heater wouldvaporize up to 100% of the diluent discharged from the reactor with thepolymer which would allow greater recovery of the diluent by theintermediate pressure condenser. Diluent recovery through the firstflash tank would reduce utility and capital cost. Conventional lowpressure single-stage diluent recovery systems include compression,distillation and treatment which have high capital and operating cost.The flash line heater would increase the temperature of the polymer inthe downstream dryer system and would create the potential for lowervolatile levels in the final product, which would lower variable cost,improves safety and aids attainment of environmental standards.

[0092] The first flash tank provides an intermediate pressure flash stepwhich allows for simple condensation of diluent and return to thereactor. The flash line heater would be capable of supplying sufficientheat to vaporize up to 100% of the diluent in the first flash tank.

[0093] Diluent vapor and unreacted/under reacted catalyst/polymer finesgo overhead from the flash tank to the cyclone. The bulk of the polymergoes out the bottom of the first flash tank through the seal chamber tothe second flash tank.

[0094] Connected to the bottom of the first flash tank is the sealchamber which provides for a low residence time plug flow area tocontrol polymer level and maintain pressure in the first flash tank. Theseal chamber is designed to accommodate a range of polymer forms fromconcentrated slurry to dry polymer.

[0095] The overhead stream from the first flash tank is received by thecyclone, which removes most of the polymer fines and returns them to thebulk of the polymer flow in the second flash tank through a two valvesystem which allows the fines to accumulate between the valves, thendischarge through the bottom valve while maintaining pressure in thefirst flash system. The overhead stream from the cyclone contains someunreacted/under reacted catalyst and polymer fines. These particles arecarried with the diluent vapor to the condenser, entrained with theliquid diluent after condensation, collected in the accumulator andreturned to the reactor in the diluent. The condensation and accumulatorsystems are designed and operated to accommodate fines.

[0096] The condenser provides for low variable and capital costliquefaction of the diluent removed from the rector with the polymer viathe first flash tank. Conventional single flash tank systems flash thepolymerization effluent to the just above ambient pressure, whichrequires compression to liquefy the diluent prior to recycle to the loopreactor. An intermediate pressure flash provides for condensation with acommonly available cooling medium, such as Plant cooling water. Thecondenser system is flushed with diluent and designed to accommodate alevel of fines without accumulation or plugging. The condenser is cooledby a tempered water system which controls the condensation temperatureto achieve the proper vapor pressure in the accumulator to allowefficient pressure control by the pressure control valve on theaccumulator vent. The condenser tempered water system is a pump-aroundloop of cooling water, the temperature of which is controlled bymetering in fresh cooling water as needed.

[0097] The accumulator receives the condensed diluent andcatalyst/polymer fines and pumps the mixture back to the loop reactorbased on level control in the accumulator. The accumulator has a bottomshape designed to accommodate fines. A vent on the accumulator purgesthe accumulated diluent of light-ends/non-condensables and controlspressure on the first flash system.

[0098] The second flash tank, operating just above ambient pressure,receives polymer from the first flash tank seal chamber. Completevaporization, if not already accomplished in the first flash tank, willoccur in the second flash tank. Polymer leaves the bottom of the secondflash tank to the dryer system. The flash-line heater would increase thetemperature of the polymer which allows the dryer system to removeresidual volatiles more efficiently and effectively. The overhead of thesecond flash tank will be diluent vapor not recovered in the first flashsystem and will be filtered and compressed for return to the loopreactor.

[0099] While the present invention has been described and illustrated byreference to particular embodiments, it will be appreciated by those ofordinary skill in the art that the invention lends itself to manydifferent variations not illustrated herein. For instance, although atwo-flash-tank system is described herein, three or more flash tanks maybe used to vaporize the desired amount of the liquid medium andsubstantially all the liquid medium will be vaporized when the effluentreaches the last flash tank. For these reasons, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

[0100] Although the appendant claims have single appendencies inaccordance with U.S. patent practice, each of the features in any of theappendant claims can be combined with each of the features of otherappendant claims or the main claim.

1. A process for producing polymer in a continuous slurry loop reactorcomprising: reacting a monomer in a hydrocarbon diluent to form apolymerization slurry of polymer solids in a liquid medium; discharginga portion of the polymerization slurry as effluent which comprises aslurry of discharged polymer solids in a discharged liquid medium,through a discharge opening into a first transfer conduit; heating theeffluent with a first heater; flashing the effluent in a first flash,wherein at least a portion of the discharged liquid medium is vaporized,to form a first flash vapor and a first flash slurry; condensing atleast a portion of the first flash vapor without compression;discharging the first flash slurry from the first flash into a secondtransfer conduit; heating the first flash slurry with a second heater;and flashing the first flash slurry in a second flash.
 2. The processaccording to claim 1, wherein the process further comprises: flashingthe first flash slurry in a second flash to form a second flash vaporand second flash polymer solids; wherein at least a portion of the firstflash liquid is vaporized in the second flash.
 3. The process accordingto claim 2 further comprising condensing at least a portion of thesecond flash vapor from the second flash.
 4. The process according toclaim 1, wherein from about 50% to about 100% of the liquid medium inthe effluent is vaporized into first flash vapor in the first flash. 5.The process according to claim 2, wherein from about 50% to about 100%of the liquid medium in the effluent is vaporized into first flash vaporin the first flash.
 6. The process according to claim 3, wherein fromabout 50% to about 100% of the liquid medium in the effluent isvaporized into first flash vapor in the first flash.
 7. The processaccording to claim 1, wherein from about 75% to about 100% of the liquidmedium in the effluent is vaporized into first flash vapor in the firstflash.
 8. The process according to claim 2, wherein from about 75% toabout 100% of the liquid medium in the effluent is vaporized into firstflash vapor in the first flash.
 9. The process according to claim 3,wherein from about 75% to about 100% of the liquid medium in theeffluent is vaporized into first flash vapor in the first flash.
 10. Theprocess according to claim 1, wherein from about 95% to about 100% ofthe liquid medium in the effluent is vaporized into first flash vapor inthe first flash.
 11. The process according to claim 2, wherein fromabout 95% to about 100% of the liquid medium in the effluent isvaporized into first flash vapor in the first flash.
 12. The processaccording to claim 3, wherein from about 95% to about 100% of the liquidmedium in the effluent is vaporized into first flash vapor in the firstflash.
 13. The process according to claim 2, wherein at least about 50%of the first flash liquid medium is vaporized into second flash vapor inthe second flash.
 14. The process according to claim 3, wherein at leastabout 50% of the first flash liquid medium is vaporized into secondflash vapor in the second flash.
 15. The process according to claim 2,wherein at least about 75% of the first flash liquid medium is vaporizedinto second flash vapor in the second flash.
 16. The process accordingto claim 3, wherein at least about 75% of the first flash liquid mediumis vaporized into second flash vapor in the second flash.
 17. Theprocess according to claim 2, wherein at least about 95% of the firstflash liquid medium is vaporized into second flash vapor in the secondflash.
 18. The process according to claim 3, wherein at least about 95%of the first flash liquid medium is vaporized into second flash vapor inthe second flash.
 19. The process according to claim 1, wherein thereactor is operated at about 150-250° F.
 20. The process according toclaim 19 wherein the reactor is operated at about 175-230° F.
 21. Theprocess according to claim 20, wherein the reactor is operated at about200-230° F.
 22. The process according to claim 1, wherein the reactor isoperated at about 400-660 psia.
 23. The process according to claim 22,wherein the reactor is operated at about 500-600 psia.
 24. The processaccording to claim 23, wherein the reactor is operated at about 565psia.
 25. The process according to claim 1, wherein the discharging intoa first transfer conduit is continuous.
 26. The process according toclaim 2, wherein the discharging into a first transfer conduit iscontinuous.
 27. The process according to claim 1, wherein the firstheater is an in-line heat exchanger.
 28. The process according to claim2, wherein the first heater is an in-line heat exchanger.
 29. Theprocess according to claim 1, wherein the first heater heats theeffluent to a temperature below the fusion temperature of the polymer.30. The process according to claim 2, wherein the first heater heats theeffluent to a temperature below the fusion temperature of the polymer.31. The process according to claim 1, wherein the first flash isoperated at about 140 to 315 psia.
 32. The process according to claim 2,wherein the first flash is operated at about 140 to 315 psia.
 33. Theprocess according to claim 3, wherein the first flash is operated atabout 140 to 315 psia.
 34. The process according to claim 4, wherein thefirst flash is operated at about 140 to 315 psia.
 35. The processaccording to claim 7, wherein the first flash is operated at about 140to 315 psia.
 36. The process according to claim 10, wherein the firstflash is operated at about 140 to 315 psia.
 37. The process according toclaim 1, wherein the second flash is operated at about 15 to 100 psia.38. The process according to claim 2, wherein the second flash isoperated at about 15 to 100 psia.
 39. The process according to claim 3,wherein the second flash is operated at about 15 to 100 psia.
 40. Theprocess according to claim 4, wherein the second flash is operated atabout 15 to 100 psia.
 41. The process according to claim 7, wherein thesecond flash is operated at about 15 to 100 psia.
 42. The processaccording to claim 10, wherein the second flash is operated at about 15to 100 psia.
 43. The process according to claim 31, wherein the secondflash is operated at about 15 to 100 psia.
 44. The process according toclaim 1, wherein the heat input to the first and second heaters isadjusted according to at least one process parameter.
 45. The processaccording to claim 2, wherein the heat input to the first and secondheaters is adjusted according to at least one process parameter.
 46. Theprocess according to claim 3, wherein the heat input to the first andsecond heaters is adjusted according to at least one process parameter.47. The process according to claim 4, wherein the heat input to thefirst and second heaters is adjusted according to at least one processparameter.
 48. The process according to claim 44, wherein the heat inputto the in-line heaters is adjusted in such a manner as to substantiallyreduce equipment pluggage.
 49. The process according to claim 47,wherein the heat input to the in-line heaters is adjusted in such amanner as to substantially reduce equipment pluggage.
 50. The processaccording to claim 44, wherein the heat input to the in-line heaters isadjusted in such a manner so as to improve the drying of the polymerproduct.
 51. The process according to claim 47, wherein the heat inputto the in-line heater is adjusted in such a manner so as to improve thedrying of the polymer product.
 52. The process according to claim 44,wherein the heat input to the in-line heaters is adjusted in such amanner so as to improve recovery of diluent.
 53. The process accordingto claim 47, wherein the heat input to the in-line heaters is adjustedin such a manner so as to improve recovery of diluent.
 54. A process forproducing polymer in a continuous slurry loop reactor comprising:reacting a monomer in a hydrocarbon diluent to form a polymerizationslurry of polymer solids in a liquid medium; discharging a portion ofthe polymerization slurry as effluent, which comprises a slurry ofdischarged polymer solids in a discharged liquid medium, through adischarge opening into a first transfer conduit; heating the effluentwith a first heater; flashing the effluent in a first flash, wherein atleast a portion of the discharged liquid medium is vaporized, to form afirst flash vapor and a first flash slurry; condensing at least aportion of the first flash vapor without compression; discharging thefirst flash slurry from the first flash into a second transfer conduit;heating the first flash slurry with a second heater; flashing the firstflash slurry in a second flash to form second flash vapor and secondflash polymer solids; and condensing at least a portion of the secondflash vapor from the second flash; wherein: about 50-100% of the liquidmedium in the effluent is vaporized into first flash vapor in the firstflash; at least 50% of first flash liquid is vaporized into second flashvapor in the second flash; the reactor is operated at about 175-230° F.;the reactor is operated at about 500-600 psia; the first flash isoperated at about 140-315 psia; the second flash is operated at about15-100 psia; and the heat input to the in-line heaters is adjusted insuch a manner as to substantially reduce equipment pluggage.